Appropriate methods for repeated surgical collection of endometrial tissue from monkeys and characterization of cytosol and nuclear estrogen and progesterone receptors have been developed. Equilibrium dissociation constants of estradiol and progesterone were 2.1 x 10 to the minus 10 power M and 3.6 x 10 to the minus 9th power M, respectively. When estrogen or estrogen plus progesterone replacement therapy was given to castrate monkeys, we found that progesterone receptor synthesis was induced by estrogens.